gh-101699: Explain using Match.expand with \g<0> (GH-101701)

Update documentation for re library to explain that a backreference `\g<0>` is
expanded to the entire string when using Match.expand().
Note that numeric backreferences to group 0 (`\0`) are not supported.

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: Stevoisiak

Doc/library/re.rst

@@ -1344,7 +1344,8 @@ when there is no match, you can test whether there was a match with a simple
    Escapes such as ``\n`` are converted to the appropriate characters,
    and numeric backreferences (``\1``, ``\2``) and named backreferences
    (``\g<1>``, ``\g<name>``) are replaced by the contents of the
-   corresponding group.
+   corresponding group. The backreference ``\g<0>`` will be
+   replaced by the entire match.
 
    .. versionchanged:: 3.5
       Unmatched groups are replaced with an empty string.